1. Field of the Invention
The present invention relates to a zoom lens adapted to a digital camera configured to obtain an image data of a subject by use of an image pickup device such as a solid-state image pickup device, or the like, more preferably, a small size zoom lens preferably adapted to a video camera configured to obtain a moving image and a digital still camera configured to photograph a still image and an information device having a photographing function by use of such a zoom lens as a photographing optical system.
2. Description of the Related Art
Recently, instead of a still camera in a film type using a silver salt film as an image recording medium, that is, a silver-salt camera, an imaging apparatus such as a digital camera obtaining a still image, a video or a moving image of a subject via a solid-state image pickup device such as a CCD (charge-coupled device) image pickup device, a CMOS (complementary metal-oxide semiconductor) image pickup device, and the like and an information device having such a camera function, for example, a portable information terminal apparatus, have been widely used. User requirements in the above devices have been diversified and particularly, of the user requirements, an imaging apparatus having a small size and high performance, and also a small size and high performance zoom lens which is used as a photographing optical system has been required.
In the zoom lens used as the photographing optical system in such an imaging apparatus, in order to downsize a zoom lens used as an imaging optical system in such an imaging apparatus, it is required to reduce an entire length of lenses when changing magnification of the zoom lens (that is, a distance from a lens surface at the most object side to an image plane), more particularly, to reduce an entire length of the lenses at a telephoto end. Furthermore, in order to downsize the imaging apparatus, it is important to suppress the entire length of the lenses at a stored state by reducing a thickness of each lens group. In order to achieve high performance of the zoom lens, considering possibility of application to the high-end digital camera, resolution corresponding to an image pickup device having at least more than 5 to 10 million pixels is required over an entire zoom range.
In order to achieve a wide filed angle of the imaging optical system, it is preferable to achieve a half field angle of 38 degree or more at a wide angle end. The half field angle of 38 degrees corresponds to a focal length of 28 mm in a case of a silver-salt camera using a silver-salt film (so-called Leica film) having a width of 35 mm.
Although there are various types of the zoom lens for a digital camera, the zoom lens having five or more lens groups is not suitable for a small size camera because it is difficult to reduce a total thickness of the lens groups.
As the zoom lens having four lens groups suitable for a high variable magnification ratio and large aperture, an example of a zoom lens including a first lens group having a positive refractive power, a second lens group having a negative refractive power, a third lens group having a positive refractive power, and a fourth lens group having a positive refractive power, which are disposed in order from an object side has been known.
In such a zoom lens adapted to high magnification ratio and a wide aperture and having four lens groups, when changing the magnification of the zoom lens from a wide angle end to a telephoto end, an interval between the first and second lens groups is increased, an interval between the second and third lens groups is decreased, an interval between the third and fourth lens groups is changed or increased, and the second lens group has a negative lens, a positive lens, and a negative lens, in order from the object side, or the third lens group includes a positive lens, a positive lens, and a negative lens, in order from the object side is disclosed in Japanese Patent Application Publication Nos. 2008-107559, 2008-112013, 2005-326743, 2008-076493, 2008-225328, 2008-026837, 2004-199000, 2008-096924.
The zoom lens disclosed in Japanese Patent Application Publication No. 2008-107559 has a wide half angle of field of about 40 degrees at the wide angle end but the magnification ratio is only about 5 times. Furthermore, the distortion at the wide angle end is about 10% and therefore even if the distortion is corrected by image processing, an image is degraded due to a large correction amount.
The zoom lens disclosed in Japanese Patent Application Publication No. 2008-112013 has a large variable magnification ratio, that is, about 9 times and a half field angle of about 40 degrees at the wide angle end as well as a relatively small telephoto ratio. However, the distortion at the wide angle end is about 10% and therefore even if the distortion is corrected by image processing, an image is degraded due to a large correction amount.
That is, the zoom lenses disclosed in both of Japanese Patent Application Publication Nos. 2008-107559 and 2008-112013 are not sufficiently suitable for a zoom lens used as a photographing optical system in a high performance digital camera, or the like.
In Japanese Patent Application Nos. 2005-326743, 2008-076493, 2008-225328 and 2008-146016, the magnification ratio is between about 2.8 times and about 6.8 times and therefore it is not sufficient for satisfying a recently-raised user requirement. In Japanese Patent Application No 2006-189598, the large magnification ratio of 15 times is provided. However, the half angle of field is about 31 degrees and therefore user requirement for a wide angle is not sufficiently satisfied. In Japanese Patent Application No. 2008-026837, the magnification ratio is about 10 times. However, the telephoto ratio is large so that compact size is not sufficiently achieved and aberration correction is required to be improved. In Japanese Patent Application No. 2008-112013, the large magnification ratio of about 9.5 times and an angle of field of about 40 degrees are provided as well as relatively less telephoto ratio. However, the aberration correction is required to be improved.
As described above, the configurations disclosed in Japanese Patent Application Publication Nos. 2008-107559 and 2008-112013 do not achieve a zoom lens having a wide angle of field and small size as well as high magnification ratio and less aberration to satisfy the user requirements. Consequently, it is required to provide a zoom lens capable of achieving a wide angle of field and a small size and having a high magnification ratio and less aberration.